Stable mode-locking region analysis of all normal dispersion optical fiber laser by measuring polarization state inside a resonator

We propose a method to find the mode-locking region of the all normal dispersion (ANDi) optical fiber laser using the transmittance map of the output coupler obtained by measuring the polarization of the beam inside the resonator. The transmittance map is a function of the rotation angles of the waveplates installed inside the ANDi fiber laser. From the correlation analysis of the measured mode-locking region and the obtained transmittance map, we found that the transmittance map can be used to the rotation angle region of the wave plates satisfying the mode-locking conditions of the ANDi fiber laser. This study is the first demonstration, to the best of our knowledge, that the mode-locking pulse occurs only under positive-feedback conditions by analyzing the relative differences in the two transmittances of the output coupler obtained using the polarization state measured in continuous wave and mode-locking modes.